1. Field of the Invention
The present invention relates generally to line drivers.
2. Description of the Related Art
Line drivers are essentially digitally-controlled variable-gain amplifiers. An exemplary application of a line driver is a return-path (i.e., upstream) amplifier in Data Over Cable Service Interface Specification (DOCSIS) certified systems (e.g., data and telephony cable modems).
Because of varying distances between a cable modem and a cable headend, DOCSIS-compliant line drivers must have the capability of applying gain or attenuation in response to a digital command signal to thereby vary the power delivered to the load impedance (typically 75 ohms) of the interconnecting communication cable.
One example of a conventional line driver is a structure that includes a preamplifier, a set of resistive attenuators and an output driver which are serially-coupled between an input signal and the load impedance. In this arrangement, the resistive attenuators change their attenuation in response to the digital command signal.
Another exemplary line driver structure replaces the set of resistive attenuators with a series combination of a vernier and a multiplying digital-to-analog converter (MDAC) that both respond to the digital command signal.
In another line driver structure, the MDAC is replaced with a pair of MDACs and the output winding of a transformer is connected across the load impedance. Each of a pair of output drivers are then coupled between a respective one of the MDACs and a respective side of the transformer""s input winding.
In yet another line driver structure, a preamplifier drives one input of a Gilbert-cell attenuator and a digital-to-analog converter responds to the digital command signal and drives another input of the Gilbert-cell attenuator. An output driver (e.g., a differential pair of transistors) then drives the load impedance in response to the output of the Gilbert-cell attenuator.
Although a variety of other line driver structures have been proposed, they and the above structures typically require output drivers whose parameters (e.g., size, power consumption,. noise and signal distortion) degrade the line driver""s performance.
The present invention is directed to line drivers that drive load impedances (e.g., communication cables) with elements that substantially improve driver performance and simplify driver structure.
These advantages are realized by directly coupling switched current mirrors to a transformer""s input winding. The mirrors are thus enabled to simultaneously provide currents in response to a differential input signal and a digital command signal and drive the load impedance (via the transformer""s output winding) to thereby realize a corresponding signal gain.